powerpc: Unsigned speed cannot be negative in udbg_16559.c

"unsigned int" speed cannot be negative, it's thus pointless
to test if it is.

Signed-off-by: Roel Kluin <[e-mail]>
Signed-off-by: Benjamin Herrenschmidt <[e-mail]>

arch/powerpc/kernel/udbg_16550.c

···
 	speed = (clock / prescaler) / (divisor * 16);
 
 	/* sanity check */
-	if (speed < 0 || speed > (clock / 16))
+	if (speed > (clock / 16))
 		speed = 9600;
 
 	return speed;